The present invention generally relates to systems for synchronizing clock frequencies in a computer network.
The Precision Time Protocol (PTP), as defined in the IEEE-1588-2008 standard entitled “Standard for a Precision Clock Synchronization Protocol for Networked Measurement and Control Systems,” enables precise synchronization of clocks over a data packet network. In a nutshell, PTP is used to distribute a “grandmaster” clock's precise time-of-day to slave clocks. This is done using an exchange of PTP packets, which include timestamps carried inside. Slave clocks evaluate these timestamps to account for end-to-end delay, and can use the resulting timing information to obtain a local time-of-day aligned to the grandmaster with sub-microsecond precision, in some cases.
The IEEE 1588 standards describe a hierarchical master-slave architecture for clock distribution. In this architecture, an “ordinary clock” is a device having a single network connection. An ordinary clock can be the source of a synchronization reference, in which case it is known as a “master.” Alternatively, an ordinary clock can be the destination of a synchronization reference, in which case it is known as a “slave.” In contrast, a “boundary clock” has multiple network connections and can transfer synchronization from one network segment to another. The root timing reference in a system is called the “grandmaster.” The grandmaster transmits time-synchronization information to slave clocks residing on its network segment. These slave clocks may include one or more boundary clocks, which can then transfer synchronized time to other clocks for which the boundary clocks serve as masters.
PTP can also be used to transfer a frequency reference over a network, i.e., to perform frequency synchronization as well as time synchronization among a grandmaster clock and one or more slave clocks. This frequency synchronization may be necessary to support a synchronous interface, such as Synchronous Ethernet (Sync-E) or the Synchronous Optical Network/Synchronous Digital Hierarchy (SON ET/SDH), for example. An International Telecommunication Union (ITU) recommendation ITU-T G.8265.1/Y.1365.1, published in October 2010, (referred to hereinafter as simply G.8265.1) contains a description of a “profile,” compatible with IEEE 1588, for frequency distribution without timing support from the network.
The use of PTP for frequency synchronization is attractive for telecommunications providers because it enables mobile backhaul to support UMTS-FDD RAN and 3G RBS. However, with PTP profile G.8265.1, PTP grandmaster ordinary clocks (GMOC) are required to provide such frequency distribution. In most cases, a Global Positioning System (GPS) receiver is required as the primary reference clock input for such a GMOC. Furthermore, the slave-only ordinary clocks (SOOC) treat two or more GMOCs independently, and no inter-GMOC communication is allowed for synchronization and redundancy control. Also, no boundary clock is allowed in this profile.
Accordingly, improved techniques for frequency distribution are needed.